Custom fields with required validation

Issues

Description

Configurable required validation for fields, so that when you create a new issue, you have to enter those fields, if toggled on.

We are moving away from the concept of "editing an issue". Instead, you should be able to change attributes to an issue independently of other attributes easily in the UI. So in this context, when you create a new issue (and required validation is turned on for a custom field), you have to fill out that field. But when you go to an existing issue, you can change anything independently. But if you change a custom field with required validation, you cannot change it to be blank. This wouldn't apply to legacy issues or fields that you subsequently made required.